Effect of metal ions on Alzheimer's disease

Abstract Alzheimer's disease (AD) is a degenerative disease of the nervous system. The typical pathological changes of AD are Aβ deposition, neurofibrillary tangles, neuron loss, and chronic inflammation. The balance of metal ions is essential for numerous physiological functions, especially in the central nervous system. More studies showed that metal ions participate in the development of AD. However, the involvement of metal ions in AD is controversial. Thus, we reviewed articles about the relationship between metal ions and AD and discussed some contradictory reports in order to better understand the role of metal ions in AD.

F I G U R E 1 Partial mechanism of metal in AD. AD may involve multiple aspects, including Aβ metabolic disorders, Tau protein hyperphosphorylation, gene mutations, oxidative stress and free radical damage, cholinergic neuron loss, inflammatory damage, and so on. Metal ions regulate AD development by participating in these physiological processes Yang et al. 2021). In the past few years, many studies have revealed a link between the pathogenesis of AD and abnormal Cu metabolism, genetic evidence suggests that the gene controlling the Cu pathway is a susceptibility gene for AD, which has been confirmed in several studies (Bucossi et al. 2012;Squitti et al. 2021;Squitti et al. 2013). Changes in Cu levels in serum, plasma, cerebrospinal fluid (CSF), and brain are associated with the development of cognitive deficits and AD (Squitti et al. 2014). Restoration of ceruloplasmin in the AD mouse brain could reduce the damage of hippocampal neurons , suggesting the neuroprotective effect of ceruloplasmin. Although most Cu in the plasma is stably bound to ceruloplasmin, some are unstable with other molecules, such as albumin and globulin. It was found that the level of non-ceruloplasmin-bound Cu (non-Cp-Cu) increased in AD and mild cognitive impairment (MCI) (Squitti et al. 2011), and it is suggested that the increase of non-Cp-Cu may be an indicator to predict the progression of MCI to AD. Further studies showed that non-Cp-Cu levels increased in the early stages of MCI. During 6 years of observation, 50% of MCI subjects with elevated non-Cp-Cu developed into AD patients within 4 years (Squitti et al. 2014).
β-Amyloid (Aβ) is produced by β-amyloid precursor protein (APP) through proteolysis of βand γ-secretase. Cu promotes the formation of Aβ plaques (Kitazawa et al. 2009). On the other hand, the Cu 2+ -Aβ complex can catalyze O 2 to produce hydrogen peroxide (H 2 O 2 ). Excessive H 2 O 2 generates a large number of free radicals through the Fenton reaction, causing a series of lipid peroxidation, protein and DNA damage. Nguyen et al. (2015) demonstrated that bis-8 (aminoquinoline) ligands could catalytically extract Cu 2+ from Cu 2+ -Aβ, the Cu-is then fully released in the presence of glutathione, forming a Cu-glutathione complex, which is an efficient biological ligand of Cu-that is able to deliver Cu ions for the formation of Cu-proteins. At present, chelating agents that can specifically bind metal ions may be an important strategy for the treatment of AD (Fu et al. 2016).
Similarly, Cu can also bind to Tau proteins and promote the formation of NFTs (Bacchella et al. 2020). In addition, Tau combined with Cu shows redox activity. Tau can reduce Cu ions and promote the generation of a series of reactive oxygen species (ROS) (Su et al. 2007). Although both Tau and Aβ are critical pathologi-cal changes in AD, the exact effects of Cu and Tau on AD have not been thoroughly studied and require further investigation to find this association.
Studies have found that Cu can increase brain inflammation and promote secretion of more proinflammatory factors, such as interleukin-1β (IL-1β), tumor necrosis factor-alpha (TNF-α), and IL-6, and downregulate the expression of LRP1 (Kitazawa et al. 2016), indicating that the inflammation promoted by Cu is one of the ways affecting the development of AD. In addition, microglia-induced neuroinflammation is closely related to AD. Cu 2+ can activate nuclear factor κB (NF-κB)-dependent microglia and produce mitochondrial ROS, and release nitric oxide (NO) and TNF-α in a time-and dose-dependent manner.
The inhibition of TNF-α or NO alone does not reduce neuronal death.
Still, the combined inhibition of TNF-α and NO could achieve this effect, so it is speculated that the combination of TNF-α and NO could cause neuronal damage (Hu et al. 2014). The application of ROS scavengers can inhibit the neurotoxicity produced by NO and TNF-α, indicating that the NO and TNF-α produced by microglia and the neurotoxicity mediated by them may be related to the mitochondrial ROS-NF-κB signal activated by Cu 2+ (Hu et al. 2014).
Cu is also involved in the synthesis of neurotransmitters (Spencer et al. 2011). Most previous studies have shown that Cu can inhibit glutamate receptor activity (Vlachova et al. 1996;Weiser and Wienrich 1996). Later, it was found that the regulation of synaptic function by Cu is not static but has a dual role: acute Cu exposure can inhibit the activity of N-methyl-D-aspartate receptors (NMDAR) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR), while chronic Cu exposure could increase the function of glutamate receptors (Bacchella et al. 2020), thus affecting learning and memory.
Cu can increase not only oxidative stress and promote the occurrence of AD by interacting with Aβ and Tau but also coregulate neural function by increasing brain inflammation and regulating synaptic function (D'Ambrosi and Rossi 2015; Hu et al. 2014;Kitazawa et al. 2016;Su et al. 2007;Spencer et al. 2011). Although the study found that Cu is closely related to Aβ and Tau pathology, the specific mechanism of action is still under further exploration.

Iron
Fe is a vital metal element in the brain that participates in oxygen transport and storage, cellular respiration, neurotransmitters, and DNA synthesis (Lane et al. 2018). Increased Fe was observed in the braindamaged area of AD patients (Maher 2018), which had a significant correlation with Aβ plaque and Tau pathology (van Duijn et al. 2017).
Ferritin is a protein that stores and regulates Fe. It is related to AD.
Elevated plasma and CSF ferritin levels are a feature of preclinical AD (Goozee et al. 2018). Elevated ferritin levels suggest elevated Fe levels in CSF and brain, which may be related to ferroptosis, which is a cell death pathway caused by lipid peroxide (Acevedo et al. 2019  Treating cells with H 2 O 2 can increase FtMt mRNA and protein levels, thus confirming that FtMt may have a neuroprotective effect on oxidative stress (Wang et al. 2011). In addition, when using Aβ25-35 to deal with FtMt knockout mice, its Bcl-2/Bax ratio decreased, and the caspase-3 level and poly ADP ribose polymerase activity and cell death increased; thus, it can be seen that FtMt deficiency can exacerbate nervous system damage caused by Aβ25-35 ).
Mitoferrin-1 is the main protein on the mitochondrial membrane that participates in transporting Fe from the cytoplasm to the mitochondria and is involved in regulating mitochondrial Fe. It is found that Mitoferrin-1 regulates Fe metabolism by changing Fe levels in the mitochondria, the expression of Fe-sulfur protein and ferritin-related genes in the Caenorhabditis elegans model of AD. Knockdown of mitoferrin-1 could reduce mitochondrial Fe content and reduce the level of mitochondrial ROS, and at the same time, Aβ reduction is also observed in the model . This shows that Mitoferrin-1 is impor- From another point of view, the above studies suggest the application prospect of Fe chelating agents in AD treatment. However, there are still many problems to be solved in practical work, such as improving the ability of metal chelating agents to pass through the bloodbrain barrier, enhancing the accuracy of Fe chelating agents, and reducing excess Fe without affecting the normal physiological function of metals, so the clinical application of Fe chelating agents still needs to be optimized.

Zinc
Zn is the second abundant trace element in human body after Fe.  Lovell et al. (1998) reported that a comparison of AD and control neuropil revealed a significant (p < .05) elevation of Zn in AD subjects and observed that Cu, Fe, and particularly Zn could accelerate the aggregation of Aβ-peptide (Lovell et al. 1998). Studies using Zn supplementation as a treatment for AD found that dietary Zn supplementation can reduce Aβ, Tau pathology, and cognitive impairment in the hippocampus of AD transgenic mice and increase the level of brain-derived neurotrophic factor (BDNF) in mouse models (Corona et al. 2010). It is well known that Aβ is produced by APP being cleaved by β-secretase and γ-secretase, and APP is cut by an enzyme with α-secretase activity, which produces the N-terminal fragment sAPPα, which is an effective neurotrophic factor. Zn treatment can reduce Tau phosphorylation and GSK-3β levels in PC12 cells induced by Aβ and reduce Aβ by lowering the activity of γ-secretase ).
This again proves the neuroprotective effect of Zn. However, some study show that high-dose zinc treatment will increases the level of APP and the activity of β-secretase, resulting in increased secretion of sAPPβ over sAPPα in the transgenic mouse brain. All of these changes promote the pathology of Aβ (Wang et al. 2010). After using Zncontaining nanoparticles to increase Zn levels in the brains of AD mice, Aβ and proinflammatory factors were significantly reduced; increased brain Zn might be beneficial rescuing some pathological alterations At present, serum Zn was significantly decreased in AD patients Ventriglia et al. 2015). The relationship between Zn and the onset of AD still needs further study, but in any case, strict control of Zn content in the body is necessary.

Manganese
Mn is a necessary nutrient element to maintain the physiological functions of the human body. In the central nervous system (CNS), Mn is an essential cofactor for several enzymes, including DNA and RNA polymerases, peptidases, carboxylases, superoxide dismutase (SOD), and glutamine synthetase (GS) (Aschner et al. 2007;Reddi et al. 2009).
A meta-study found that the serum Mn levels are lower in AD patients, and Mn deficiency may be a risk factor for AD , showing some kind of connection between Mn and AD. However, various organs will be damaged after excess Mn exposure, especially the CNS, resulting in a neurodegenerative disease affecting cortical structures and basal ganglia (Dobson et al. 2004).
The cholinergic theory is widely studied, and currently, there are drugs for the treatment of AD according to it. The basal forebrain is a crucial central cholinergic region, establishing the SN56 cells as the basal forebrain cholinergic neuron model to study the toxicity mechanism of neurons after Mn exposure. The study found that after acute and long-term Mn exposure, acetylcholine levels decreased, acetylcholine transferase activity decreased and acetylcholinesterase activity increased. It is well known that BDNF can promote the survival of cholinergic neurons, which is closely related to synaptic plasticity and affect people's learning and memory functions. However, Mn exposure can reduce BDNF expression in the rat hippocampus . A population study found that occupational Mn exposure reduced the plasma BDNF and cognitive ability of the population, and the degree of BDNF decline was positively correlated with the degree of cognitive impairment (Zou et al. 2014).
It is well known that NF-κB is related to the activation of glial cells and the production of inflammatory factors (Kirkley et al. 2017). Glial cells are the main target of Mn. Mn can increase the number of inflammatory factors produced by NF-κB-regulated microglia and astrocytes (Chen et al. 2006;Spranger et al. 1998). Inhibiting NF-κB in glial cells has anti-inflammatory and neuroprotective effects. In-depth studies have found that the expression of inflammatory genes regulated by NF-κB in Mn-treated mixed glial cells (microglia and astrocytes) is significantly higher than in single microglia or astrocytes. The survival rate of neurons in mixed glial cell culture fluid exposed to Mn was significantly lower than in single glial cell culture fluid. This indicates that the interaction between microglia and astrocytes can produce more destructive inflammatory mediators and enhance the neuroinflammatory damage caused by Mn exposure (Popichak et al. 2018).
Mn blocks APP and heavy-chain ferritin protein translation in a dose-and time-dependent manner, leading to the accumulation of Fe 2+ . Increasing APP expression can partially reduce Mn-induced ROS production and neurotoxicity (Rogers et al. 2019;Venkataramani et al. 2018). On the other hand, Mn can also weaken the body's antioxidant defense. Mn treatment significantly increased intracellular ROS and malondialdehyde levels, while GSH levels, SOD, and GPX4 activity were significantly reduced. Antioxidants applied to Mn-treated cells were able to reverse these results (Bahar et al. 2017). The increase of Mn concentration initially promoted the increase of oxidative stress.
The increased oxidative stress in the body can further lead to the imbalance of Fe 3+ and Fe 2+ homeostasis, which in turn induces a variety of Fe-mediated neuronal damage mechanisms, thereby exaggerating the Mn-induced neurodegeneration (Fernsebner et al. 2014).
Astrocytes are the most abundant glial cells in the brain and are vital for normal brain function, one of the functions of astrocytes is to reg-ulate synaptic activity and maintain glutamate levels. Glutamate levels are increased by the accumulation of Mn in the brain (Fernsebner et al. 2014). This may be due to the GS associated with GS in astrocytes, and Mn inhibits the glutamate transporter related to glutamate uptake. All of these results in elevated glutamate levels that mediate neuroexcitatory toxicity have been shown to be connected with various neurodegenerative diseases (Deng et al. 2012;Lee et al. 2017).
The Mn pollution in the environment is becoming more serious.
Although MN is an essential metal element for the human body, too much Mn exposure can disrupt normal nerve function and participate in AD through neuroinflammation, oxidative stress, neuronal loss, and regulation of neurotransmitters. The mechanism of this has yet to be further verified.

CONCLUSIONS
The current study found that the pathogenesis of AD may involve multiple aspects, including Aβ metabolic disorders, Tau protein hyperphosphorylation, gene mutations, oxidative stress and free radical damage, cholinergic neuron loss, inflammatory damage, and so on. It is also because of the multiple pathways of AD pathogenesis that it is challenging to develop drugs for AD. Existing drugs can only improve symptoms to a certain extent, and there is a lack of drugs that can prevent the disease process or reverse its pathophysiological process. A single factor does not cause AD. It is very challenging to design drugs that target multiple areas without losing their specificity. A detailed understanding of its physiological regulation process, cellular and molecular mechanisms, and its changes in AD may be able to provide help for precision treatment. Metal-containing protein may be beneficial or harmful. How to adjust the balance needs to be noticed in metal chelator research. Since the potential dangers of metals are known, how can they be prevented in daily life? With the continuous improvement of our understanding of AD, our treatment should be more targeted.
It may be more beneficial to choose different disease stages of AD patients or even distinguish specific types and stages of metal imbalance patients.

ACKNOWLEDGMENT
This work was supported by Hebei Provincial Department of Finance 2018-2020 project (361004) and Natural Science Foundation of Hebei Province (H2020206631).

CONFLICT OF INTEREST
The authors report no conflict of interest.

AUTHOR CONTRIBUTIONS
F. L. have made substantial contributions to conception and design; Z.

CONSENT FOR PUBLICATION
The manuscript is not submitted for publication or consideration elsewhere.

DATA AVAILABILITY STATEMENT
All data generated or analyzed during this study are included in this published article.

PEER REVIEW
The peer review history for this article is available at https://publons. com/publon/10.1002/brb3.2527